Turboprop climb/descent profiles, block time vs fuel efficiency

Roger Roger

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So I’ve kind of been pondering this for a while and I figure it might make a decent thread in the spirit of the technical talk subforum.

For a turboprop aircraft, what generally is the most fuel-efficient climb profile? Slower airspeed/higher rate to get high and get the fuel burn down sooner, or burn more gas/time unit and get downrange faster? Assume we’re running max torque/temp and calm winds. What about time-efficient? Does a faster IAS in climb pay off despite the longer amount of time spent in climb? Is there a rule of thumb one could use like x% of Vy?

Similar questions on descent. What’s more fuel efficient? Staying high and divebombing at Mmo/Vmo to keep the fuel burn down, or starting down early and staying at a slower speed that is more aerodynamically efficient? How much power to use in descent? Would a true zero-torque descent power be most efficient? I would think getting into a negative torque situation would be less efficient as you’re creating an airbrake. Are turboprop torque indications accurate enough near zero to set a true zero thrust/zero drag condition?

How much of this is airframe dependent? That’s basically what I got from Aerodynamics for Naval Aviators-that turboprop engine and airframe configurations vary so much they didn’t write rules of thumb the way they did with turbojet.
 
To be honest the entire two years of flying turbo props I never worried about fuel efficiency, except when trying to takeoff from JFK when you single engine taxied a turboprop with no nose wheel steering that was never designed to taxi single engine.

it was always Max ITT for the power settings and Vmo for the airspeed. Vmo for approach and landing on the glide slope to 3 mile final.

For climb out I think it was 140kts to get up to cruise as fast as possible or 180kts for cruise climb to go as fast as you could while still having a positive climb rate. Or 160kts between the two extremes.

if I knew I had a tail wind at cruise, I’d fly 140 to try to get into the tailwinds as soon as I could. If I knew I was going to get into headwinds, then I’d fly 180.

probably (definitely?) not the most efficient way to fly and/or not the most comfortable flight for the passengers.

fuel efficiency was never a thought or goal.
Fuel for a typical 20-40 min flight segment was always about 1200-1600lbs including the 45min reserve. At Max payload Max fuel you could take was 1900-2000lbs

But I had no idea how little I knew (basically nothing) about flying the first two years and 2400hrs of my career until I left the turboprop airline and in hindsight and every time I reflect on that time in my career am always amazed that I didn’t make a smoking hole in the ground during that time.

TL-DR.... I guess you could try to find the fuel efficient profile, but how much would/could you save? When the amount of fuel being discussed in total for an entire flight amounts to what is taxi fuel for a turbojet.
 
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Some considerations for the climb...

Altitude over Airspeed:
-Blackhawk marketing shows how quickly the -67 KA350 will get to altitude and into the thin air.
-ATC might get you on course quicker when you reach altitude

Airspeed over Altitude:
-passenger comfort (EMS)
-leg length and final altitude

I known you were discounting wind, but you really can’t discount the wind. Are you climbing into a 100 kt headwind or tailwind? Of course, I’d want to capture that big push ASAP.

For the descent, the approach of staying as high as long as possible works. You can even delay the descent beyond the planned 300/1 to 1500 AGL/5nm prior to the airport. Use 400 or more — just watch the 91.117 limits. In EMS, this was more challenging because the patient’s head was forward and holding a prolonged nose-down attitude could make things uncomfortable.

I flew with one pilot who did the 1500/5 prior without fail, until I mentioned we’d be going another couple of miles beyond the airport to come back in to land. There’s another 5 miles of low fuel flow. If you’re flying with an FMS, you’ll see your fuel at destination increase during the descent. I’d think the idle vs zero thrust would be airframe specific, ie RX-12X vs. Piaggio.

Didn’t AirNet have an idle descent profile for their Lear 35s?
 
To be honest the entire two years of flying turbo props I never worried about fuel efficiency, except when trying to takeoff from JFK when you single engine taxied a turboprop with no nose wheel steering that was never designed to taxi single engine.

it was always Max ITT for the power settings and Vmo for the airspeed. Vmo for approach and landing on the glide slope to 3 mile final.

For climb out I think it was 140kts to get up to cruise as fast as possible or 180kts for cruise climb to go as fast as you could while still having a positive climb rate. Or 160kts between the two extremes.

if I knew I had a tail wind at cruise, I’d fly 140 to try to get into the tailwinds as soon as I could. If I knew I was going to get into headwinds, then I’d fly 180.

probably (definitely?) not the most efficient way to fly and/or not the most comfortable flight for the passengers.

fuel efficiency was never a thought or goal.
Fuel for a typical 20-40 min flight segment was always about 1200-1600lbs including the 45min reserve. At Max payload Max fuel you could take was 1900-2000lbs

But I had no idea how little I knew (basically nothing) about flying the first two years and 2400hrs of my career until I left the turboprop airline and in hindsight and every time I reflect on that time in my career am always amazed that I didn’t make a smoking hole in the ground during that time.

TL-DR.... I guess you could try to find the fuel efficient profile, but how much would/could you save? When the amount of fuel being discussed in total for an entire flight amounts to what is taxi fuel for a turbojet.
Of course it’s nitpicky and one vector down at 3000 will throw all of it out the window, it’s just the kind of thing I ponder at 3 AM watching the top of descent creep closer on the moving map. We do also on occasion have a leg length/wind combination that pushes the airplane especially if you need an alternate.Anecdotally climbing at the manufacturer recommended profile (fast) a Vy profile (slow) and a company-recommended profile (in between) seems to have no detectable difference in block times. Despite manufacturer claims if it’s warm and I’m heavy I’m usually at Vy the last couple thousand feet anyway.
Some considerations for the climb...

Altitude over Airspeed:
-Blackhawk marketing shows how quickly the -67 KA350 will get to altitude and into the thin air.
-ATC might get you on course quicker when you reach altitude

Airspeed over Altitude:
-passenger comfort (EMS)
-leg length and final altitude

I known you were discounting wind, but you really can’t discount the wind. Are you climbing into a 100 kt headwind or tailwind? Of course, I’d want to capture that big push ASAP.

For the descent, the approach of staying as high as long as possible works. You can even delay the descent beyond the planned 300/1 to 1500 AGL/5nm prior to the airport. Use 400 or more — just watch the 91.117 limits. In EMS, this was more challenging because the patient’s head was forward and holding a prolonged nose-down attitude could make things uncomfortable.

I flew with one pilot who did the 1500/5 prior without fail, until I mentioned we’d be going another couple of miles beyond the airport to come back in to land. There’s another 5 miles of low fuel flow. If you’re flying with an FMS, you’ll see your fuel at destination increase during the descent. I’d think the idle vs zero thrust would be airframe specific, ie RX-12X vs. Piaggio.

Didn’t AirNet have an idle descent profile for their Lear 35s?
Pax comfort-the Pilatipus doesn’t really have enough grunt to get an uncomfortable deck angle.
Certainly where we operate the first 6000’ it’s important to get up to MVA so we can get a direct on course.

On descent I’ve fiddled with it a bit and you can easily do 5-6° with no tailwind. You’ll lose the vertical track below you for the first 6-8k feet because you’re MMO limited but once you’re down in the teens if you keep it Vmo minus 10-15 depending on turbulence you’ll catch it by 5-6k easy. Patient comfort could definitely become an issue between the nose down attitude and hauling butt through any turbulence (as you know the PC rides bumps like an empty 1-ton pickup).

Thinking about power settings, I guess you would actually want a small positive torque because you want the engine to be doing the work of turning the prop, otherwise it’s a big drag device and that would kill fuel efficiency. But then again maybe not? What does the fuel flow vs torque curve look like? You’re burning x pph just to keep the engine running, maybe bumping it up to x+50 actually gives you a return? On one hand it means you spend more time lower, on the other it means you spend longer at your descent speed. But then again, you’re usually cruising fast enough that you don’t gain a lot of TAS in the descent. So maybe absolute minimum fuel flow while still letting the engine drive the prop is the name of the game. I guess it depends on airframe/engine rigging-the Caravans I flew both Pratt and Garrett would definitely give you drag if you pulled enough power. Something about the way the Pilatus installation is rigged it doesn’t feel like it does, except momentarily coming into the landing flare.

It’s interesting to me because there’s all kinds of stuff out there about turbojets because airlines analyze it down to the smallest decimal place but turboprops there’s a much higher bias toward “eh, whatever helps you fit in with traffic”. Which of course isn’t as much of a concern at 0300 or going into a 1 in, 1 out airport. It is fun on short legs (for example yeeting across the cascades from BFI to YKM or EAT) to see how high you can get on a parabolic type altitude profile and see if you can beat the fltplan fuel numbers.

I also wonder at what point of bypass ratio turbofans start to act more like turboprops from a fuel efficiency standpoint. Or maybe straight wings vs swept matters more. But probably not, because most turbojet stuff is based on the idea that fuel flow is related to thrust production, and at some point the bypass ratio becomes so high that the performance of the fan at different altitudes and airspeeds starts to be more propeller-like.
 
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No science in my approach, I just use back of the napkin, rule of thumb. If there is a tailwind to be had, I climb quicker, around 120, and a head wind, 150 or a touch more. Tail wind on decent 3X altitude in miles. Headwind maybe 4X or more.
 
For efficiency, think of it in terms of energy exchange and how efficiently that exchange can occur and a few things fall out:
  • Engines are a source of mechanical energy to moving the whole system. They operate by converting chemical energy so you want to operate at the maximum thermal efficiency of the engine. This is almost always the design point at 100% power (no afterburner), so when you're putting energy into the system, do it at full power.

  • Energy from the engines goes to displacing the aircraft, both through the air and in the gravitational potential. As the aircraft ascends, mechanical energy becomes potential energy that can be exchanged later without loss. The energy spent exchanging momentum with the air (lift and drag) is gone once it's exchanged. So if you have to do that, do it efficiently. Climb at the proscribed speeds at maximum power and convert whatever excess energy there is into potential by climbing at whatever rate results. Something like L/D max at full power will be the speed (faster than holding, slower than Vmo). Drag is higher at faster speeds and a with a larger lift force.

  • The only time you want to weigh a lot is at the top of descent. Instead of chemical energy, you are converting potential energy to continue exchanging momentum with the air, so why not have a lot of energy at the top. Idle descent at the L/D max speed for idle thrust all the way down is usually the most efficient path. Where top of decent is located depends on how high the aircraft is and how much it weighs.
These are more "guidelines for any aircraft" than anything specific to turboprops. As you speculated, I would also imagine there is a rule-of-thumb for idle descent about about keeping torque just of the bottom peg; you're either losing energy from chemical energy in the fuel or momentum exchange with air, maybe fuel is the more efficient option(?).
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It’s interesting to me because there’s all kinds of stuff out there about turbojets because airlines analyze it down to the smallest decimal place but turboprops there’s a much higher bias toward “eh, whatever helps you fit in with traffic”.

This is also true. :D I think it comes from the fact that turboprops are just so stinking efficient compared to jets for equivalent missions.

For example, it is valuable to jets to get to altitude rapidly because their efficiency at cruise is just so much better up there. But I am not sure that is as critical for turboprops(?).
 
We fly mostly at 330 east bound and try to get up there as quick as possible then ride the wind. That climb starts at 160 then slows down to about 135 if it’s warm up there. West bound if the winds are strong I’ll climb at 160 until cruise which is usually 260 or 280.
 
So I’ve kind of been pondering this for a while and I figure it might make a decent thread in the spirit of the technical talk subforum.

For a turboprop aircraft, what generally is the most fuel-efficient climb profile? Slower airspeed/higher rate to get high and get the fuel burn down sooner, or burn more gas/time unit and get downrange faster? Assume we’re running max torque/temp and calm winds. What about time-efficient? Does a faster IAS in climb pay off despite the longer amount of time spent in climb? Is there a rule of thumb one could use like x% of Vy?

Similar questions on descent. What’s more fuel efficient? Staying high and divebombing at Mmo/Vmo to keep the fuel burn down, or starting down early and staying at a slower speed that is more aerodynamically efficient? How much power to use in descent? Would a true zero-torque descent power be most efficient? I would think getting into a negative torque situation would be less efficient as you’re creating an airbrake. Are turboprop torque indications accurate enough near zero to set a true zero thrust/zero drag condition?

How much of this is airframe dependent? That’s basically what I got from Aerodynamics for Naval Aviators-that turboprop engine and airframe configurations vary so much they didn’t write rules of thumb the way they did with turbojet.
Jet engines accelerate a little air a lot. Prop engines accelerate lot of air a little. ...I have no idea if that's helpful.

It's a turbo-PROP. You're thinking too hard. Carry on. ;)
 
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In most turbo props, climb and descent profiles seem to make little difference beyond 3 minutes.
Descent against head winds sooner, Descent with a tail wind later.

if you're making all sorts of calculations to make an extended leg, save yourself time and add a fuel stop.
 
I think the question of whether cruise climb or Vy climb is better for fuel is very much more "Cart" dependent than "Horse." For instance, I fly a very heavy, and very draggy KA 350, and to fly max range cruise in the mid-20s it takes about 80pph more than the book says for a straight 350 even when we've burned way down. That means the fuel burn margin between a max temp climb and a max range cruise is pretty small for me. On another plane, the margin may be huge, and made even more so by a long cruise leg. On top of that, for most turboprops that lose oomph in the high teens when they're heavy, all climbs intercept the same profile at some point as you're trying to chug that last several thousand feet rendering the time you have to gain an advantage to just a few minutes at the beginning of the flight.
 
Didn’t AirNet have an idle descent profile for their Lear 35s?

Not sure about Airnet, but I could believe it because the fuel burn on those Lears was unholy and they didn’t carry that much. This is all anecdotal, I flew with a guy who had flown early Lears. Rocket up, cruise, fall out of the sky because that was the only way it worked. But those planes had the performance to do it too.
 
We had profiles for the Dash 8, but nothing like what our 145 ops are. I was taught by old turboprop drivers to climb as fast as you can just to get out of the way. 160-170 indicated climb (Dash 8-200/300). Descent was when you flew to the barber pole to gain the speed you couldn’t have in climb or cruise. And as someone mentioned, whatever you could do to fit into the traffic flow. NY approach hated us climbing but loved squeezing us into the landing flow.

But honestly, fuel burn in a turboprop wasn’t a huge deal like a jet and even now, we have people (including check airman) who don’t teach or follow the climb schedule in our 145’s and claim it all works out in the end. Most of them have much more 145 time than I do, but I follow as close to our climb schedule as I can and don’t usually have fuel issues.

I recall the Dash at takeoff power and initial climb was about 1200 pph/per engine. The 145 doubles that number easily. Granted that the 145 is faster and climbs faster, but it’s appetite for fuel is huge compared to a turboprop.
 
From a strictly performance math side of the equation, it’s a variable speed in the climb, and generally not worth the time spent calculating it unless you are bored. Thankfully the OEM kinda does that for us and gives us book numbers, which will pretty much ballpark your best climb profiles.

If you want to clean it up, pay attention to wasted movement. If departing the wrong way, minimize airspeed, maximize rate of climb until on course. If a DP includes altitude based turns, then, go for that.

From an operator perspective, engines almost always cost more. Airline is different, but an operator paying power/hour ESP rates to Pratt etc, until fuel hits nearly $7 a gallon, the engine almost always cost more than the gas- so do what gets you there faster.


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